1. Field of the Invention
This invention relates to a wavelength division multiplexing optical transmitter and a wavelength division multiplexing-demultiplexing optical transmission-reception system to which the wavelength division multiplexing optical transmitter is applied.
2. Description of the Related Art
Long haul large capacity submarine optical communication systems which make use of optical amplification have already been put into practical use. In order to further increase the capacity, much effort is being directed to development of an optical wavelength division multiplex (WDM) system to put it into practical use by various organizations for research and development.
In order to put the WDM system into practical use, it is essentially required to make investigations for a main transmission line and particularly to suppress the signal to noise ratio (SNR) when the WDM system is implemented.
As a factor which deteriorates the optical SNR peculiar to the WDM system described above, beat noise arising from four-wave mixing is generally known, and for an optical wave multiplexer or demultiplexer to be developed for use with a WDM system, it is required to suppress the four-wave mixing just mentioned.
By the way, it is known that beat noise arising from four-wave mixing mentioned above can be suppressed by arranging, when to multiplex and transmit a plurality of optical signals, the wavelengths of the optical signals to be multiplexed and transmitted in an unequally spaced relationship from each other.
Accordingly, it has been examined to employ, as an optical wave multiplexer or demultiplexer to be used by the WDM system described above, an optical wave multiplexer or demultiplexer which includes, for example, a plurality of filters which extract optical signals having wavelengths arranged in an unequally spaced relationship from each other to allow an unequally spaced arrangement of optical wavelengths.
With the system which suppresses beat noise using such an optical wave multiplexer or demultiplexer as described above, however, a plurality of filters which extract optical signals having wavelengths arranged in an unequally spaced relationship from each other are used, and it is difficult to finely adjust the wavelengths of light passing through the filters.
Accordingly, the system described above has a subject to be solved in that it is difficult to obtain required characteristics for multiplexed light signals (that the wavelengths are arranged in an unequally spaced relationship from each other). Besides, a filter which is high in accuracy must be used for the filters, which makes an obstacle to reduction in cost.